Materials which retain their thermomechanical properties, particularly strength and thermal shock resistance, even under extremely reducing conditions, are required in process engineering and particularly in melt metallurgy.
Although ceramics based on pure aluminium titanate or tialite show interesting properties, such as a low thermal expansion coefficient (TEC) and a low Young's modulus, they are of only limited technological value on account of their very poor strength and their tendency to decompose at temperatures in the range from about 900.degree. to 1300.degree. C. At temperatures in this range, tialite decomposes into the starting oxides Al.sub.2 O.sub.3 and TiO.sub.2, accompanied by a pronounced increase in the TEC and a correspondingly inadequate thermal shock resistance.
Numerous attempts have been made to counteract this tendency towards decomposition. Thus, U.S. Pat. No. 2,776,896 refers to the stabilizing effect of iron solid solutions in Al.sub.2 TiO.sub.5. SiO.sub.2 and MgO also show a stabilizing effect, although it is not as pronounced as in cases where the preferred Fe.sub.2 O.sub.3 is added. However, it is pointed out that additions of SiO.sub.2 and MgO can be used for stabilization when the electrical properties, the color or the susceptibility to reduction of the iron-containing aluminium titanate are not wanted.
The superiority of iron stabilization is also described in DE-A 3 814 079, according to which an adequate Fe.sub.2 O.sub.3 concentration is responsible for long-term stabilization.
This no longer applies when the stability of aluminium titanate under strongly reducing or varying atmospheres is investigated. Thus, an Al.sub.2 TiO.sub.5 ceramic stabilized solely with additions of Fe.sub.2 O.sub.3 shows signs of decomposition in a pure CO atmosphere at a temperature of 1050.degree. C.
The problem addressed by the present invention was to provide an inexpensive aluminium titanate ceramic which would show sufficient resistance to decomposition at temperatures of .gtoreq.1000.degree. C. both under strongly reducing conditions (for example CO) and under oxidizing conditions (for example air) to retain its thermomechanical properties, particularly its strength and thermal shock resistance.
There are only limited references in the prior art to improving the susceptibility of aluminium titanate ceramic to reduction. Thus, U.S. Pat. No. 4,118,240 describes additions of 1.5 to 10% by weight SnO.sub.2 and 2 to 13% by weight SiO.sub.2 or 0.5 to 10% by weight rare earth oxide and 2 to 13% by weight SiO.sub.2. Samples are said to show improved resistance to decomposition, even under reducing conditions. Flexural strength at room temperature is only of the order of 18 to 35 MPa.
Although there is much more patent literature relating to SiO.sub.2 --, MgO-- and Fe.sub.2 O.sub.3 -- containing aluminium titanate ceramic, there are no apparent solutions to the problem stated above.
EP-B 37 868 describes an aluminium titanate ceramic containing 1 to 20% by weight SiO.sub.2 (preferably 2 to 15% by weight), 1.2 to 20% by weight MgO (preferably 2 to 17% by weight) and 0.5 to 20% by weight Fe.sub.2 O.sub.3 (preferably 2 to 10% by weight). Flexural strengths of only 7 to 35 MPa are measured at room temperature.
EP-A 210 813 describes a low-glass aluminium titanate/mullite ceramic containing at least one presynthetized component, namely aluminium titanate and/or mullite. The ceramic contains at least 1.2% by weight Fe.sub.2 O.sub.3. For a maximum MgO content of 0.8% by weight, the MgO:Fe.sub.2 O.sub.3 ratio is .gtoreq.0.67. Resistance to decomposition under highly reducing conditions is not guaranteed.
DD-B 29 794 claims an aluminium titanate ceramic containing additions of 0 to 40% by weight SiO.sub.2 and 0 to 20% by weight MgO. In addition, 0.05 to 15% by weight of the oxides Zn, Ca, Ba, Fe, Ni, Cu, Mn and Cr may be added. In Example 4, 20% by weight MgO and 1.5% by weight Fe.sub.2 O.sub.3 are added, which gives a ceramic of insignificant strength.
DE-C 2 750 290 describes a silicate-containing aluminium titanate ceramic produced from a raw material mixture containing 2 to 5% by weight kaolin and 0.1 to 1% by weight magnesium silicate. Under the assumption that the magnesium silicate is sepiolite (column 4, line 36), this corresponds to an SiO.sub.2 content of 1.0 to 2.9% by weight and to an MgO content of 0.025 to 0.25% by weight. The material calcined at temperatures of 1350.degree. to 1450.degree. C. has strengths of 30 to 40 MPa (Examples 4 and 5), a Young's modulus of approximately 13 GPa and a TEC (measured in the range from 25.degree. to 1000.degree. C.) of 1.3 to 1.5.times.10.sup.-6 l/K. There are no quantitative references to the resistance to decomposition under reducing or oxidizing conditions.
DE-A 3 644 664 describes an aluminium titanate ceramic containing four additives, namely: 2.5 to 3.0% by weight SiO.sub.2, 0.5 to 1.0% by weight MgO, 0.1 to 1.5% by weight Fe.sub.2 O.sub.3 (preferably 0.5 to 1% by weight) and 0.1 to 2.5% by weight La.sub.2 O.sub.3 (preferably 0.5 to 2.0% by weight). The Example contains 2.56% by weight SiO.sub.2, 0.74% by weight MgO, 0.74% by weight Fe.sub.2 O.sub.3 and 1.96% by weight La.sub.2 O.sub.3. After heating for 100 h at 1100.degree. C. (there is no mention of the atmosphere), this ceramic still contains 61% Al.sub.2 TiO.sub.5. However, this ceramic is not sufficiently resistant to decomposition under reducing conditions.